1. Field of the Invention
The present invention relates to a GaN green LED drive device and an optical transmission device and particularly to a GaN green LED drive device for driving a GaN green LED (light-emitting diode) at a high speed and an optical transmission device using the GaN green LED drive device.
2. Description of the Related Art
An optical transmission device for transmitting and receiving an optical signal through a plastic optical fiber (hereinafter referred to as “POF”) which is an optical transmission medium is used in relatively short-distance (not longer than 100 m) optical communication such as inter-device optical communication. The optical transmission device includes a light-emitting device for generating an optical signal, and a light-receiving device for receiving an optical signal from another optical transmission device.
Generally, an AlGaInP red LED is used as the light-emitting device. The AlGaInP red LED can make high-speed response. The AlGaInP red LED, however, has a problem that the emitted light output is reduced greatly in accordance with the temperature change. For example, the wavelength of the emitted light output is reduced by about 20 nm when the temperature change is 100° C. For this reason, transmission loss in the POF varies largely, so that the transmission distance is limited to about 50 m.
On the other hand, a GaN green LED (wavelength: 520 nm) for emitting green light in a low-loss wavelength range in the POP has come onto the market for the display purpose in recent years. It was conceived that the GaN green LED could transmit light by a distance of 100 m or longer, because green light in a low-loss wavelength range in the POF is emitted as well as because the GaN green LED has such characteristic that lowering of the emitted light output and fluctuation of the wavelength due to the temperature change are smaller as compared with the red LED. The GaN green LED, however, has a problem of being bad in trailing edge characteristic.
FIG. 7A is a waveform graph of a conventional bias current supplied to the GaN green LED. The bias current is shaped like a pulse to obtain a digital optical signal. The low level of the bias current is about 0 mA whereas the high level of the bias current is 20 mA. FIG. 7B is a waveform graph of a peaked bias current. FIG. 7C is a waveform graph of a light output emitted from the GaN green LED. As shown in FIG. 7C, the trailing edge of the emitted light output is sharp just after the beginning of the fall but becomes slower with the passage of time after the beginning of the fall. Hence, the trailing edge characteristic of the GaN green LED is very bad.
FIG. 8 is a graph showing waveforms for measuring the fall time of the GaN green LED. In FIG. 8, the upper trace part shows a waveform of a bias current supplied to the GaN green LED, and the lower trace part shows a waveform of a light output emitted from the GaN green LED. As shown in FIG. 8, the trailing edge characteristic of the light output in a digital operation of the GaN green LED is about 38 nsec in terms of the fall time t, required for changing the output from 90% to 10%. In this case, the transmission speed of an optical signal outputted from the GaN green LED is limited to about 20 Mbps. Hence, the GaN green LED cannot be applied to a high-speed optical communication device;